221.  Wool and Silk Bleaching.  Animal fibers like silk, wool, and feathers, and some vegetable fibers like straw, cannot be bleached by means of chlorine, because it attacks not only the coloring matter but the fiber itself, and leaves it shrunken and inferior.  Cotton and linen fibers, apart from the small amount of coloring matter present in them, contain nothing but carbon, oxygen, and hydrogen, while animal fibers contain in addition to these elements some compounds of nitrogen.  The presence of these nitrogen compounds influences the action of the chlorine and produces unsatisfactory results.  For animal fibers it is therefore necessary to discard chlorine as a bleaching agent, and to substitute a substance which will have a less disastrous action upon the fibers.  Such a substance is to be had in sulphurous acid.  When sulphur burns, as in a match, it gives off disagreeable fumes, and if these are made to bubble into a vessel containing water, they dissolve and form with the water a substance known as sulphurous acid.  That this solution has bleaching properties is shown by the fact that a colored cloth dipped into it loses its color, and unbleached fabrics immersed in it are whitened.  The harmless nature of sulphurous acid makes it very desirable as a bleaching agent, especially in the home.

Silk, lace, and wool when bleached with chlorine become hard and brittle, but when whitened with sulphurous acid, they retain their natural characteristics.

This mild form of a bleaching substance has been put to uses which are now prohibited by the pure food laws.  In some canneries common corn is whitened with sulphurous acid, and is then sold under false representations.  Cherries are sometimes bleached and then colored with the bright shades which under natural conditions indicate freshness.

Bleaching with chlorine is permanent, the dyestuff being destroyed by the chlorine; but bleaching with sulphurous acid is temporary, because the milder bleach does not actually destroy the dyestuff, but merely modifies it, and in time the natural yellow color of straw, cotton, and linen reappears.  The yellowing of straw hats during the summer is familiar to everyone; the straw is merely resuming its natural color which had been modified by the sulphurous acid solution applied to the straw when woven.

222.  Why the Color Returns.  Some of the compounds formed by the sulphurous acid bleaching process are gradually decomposed by sunlight, and in consequence the original color is in time partially restored.  The portion of a hat protected by the band retains its fresh appearance because the light has not had access to it.  Silks and other fine fabrics bleached in this way fade with age, and assume an unnatural color.  One reason for this is that the dye used to color the fabric requires a clear white background, and loses its characteristic hues when its foundation is yellow instead of white.  Then, too, dyestuffs are themselves more or less affected by light, and fade slowly under a strong illumination.